Solvent dewaxing or autorefrigerative dewaxing of waxy hydrocarbon oils, especially waxy lubricating oils, transformer oils, specialty oils, white oil or even certain fuels fractions such as jet or kerosene result in the recovery of substantial quantities of wax. Fischer-Tropsch synthesis also is a route to the formation of substantial quantities of wax. In many instances, especially when the waxes are directly synthesized by the Fischer-Tropsch route, the waxes are characterized by being predominantly, i.e., about 75%, normal, straight chain paraffins which, while marked by having high viscosity indices are equally marked by having high melting points.
These waxes, however, although undesirable in oils or fuels, are themselves valuable products when used in specialized applications such as in the production of food containers, waxed papers, candles, etc.
Before these predominantly n-paraffin waxes can be employed in such applications, it is necessary to separate them into various fractions, i.e. softer, lower molecular weight waxes melting at lower temperature, and harder, higher molecular weight waxes melting at higher temperature. In this way specialized fractions especially well suited for particular applications are secured.
In the past these fractions have been recovered by distillation, which is an energy intensive operation or by melting followed by recrystallization. Recrystallization can be accomplished either in the presence or absence of a solvent such as MEK, MIBK, Toluene, mixtures of such solvents, etc.
It would be an improvement if the mixtures of n-paraffin waxes could be separated into light fractions and heavy fractions by a procedure less energy intensive than distillation and more selective than melting/recrystallization.
U.S Pat. No. 4,874,523 teaches a method for reducing the cloud point of materials, especially waxy hydrocarbon oils, by use of an ultrafiltration separation technique. The method comprises the steps of chilling the waxy oil so as to achieve submicron crystallization followed by ultrafiltration thereby effecting separation of a permeate of reduced cloud point from a retentate. Chilling to produce submicron p;crystallization is to a temperature about 3.degree. C. or less above the cloud point of the material to be separated, preferably from 0.5.degree. to 2.degree. C. above the cloud point, most preferably between 1.degree. to 2.degree. C. above the cloud point. The membrane used is any ultrafiltration membrane which is inert in and non-reactive with the components of the solution, mixture or melt being separated, will not itself melt at the temperatures employed and possesses an effective pore size smaller than the submicron crystals formed upon chilling. The membrane also does not have a strong chemical affinity for the crystallized components of the feed mixture.